A Long Term Evolution (LTE) system adopts carrier aggregation (CA) that uses a predetermined bandwidth (a maximum of 20 MHz) as a basic unit and performs communication using a plurality of carriers at the same time.
When CA is performed, a primary cell (PCell) that has high reliability and secures connectivity and a secondary cell (SCell) which is a subsidiary cell are configured in user equipment. SCell is a cell that is added to PCell and is configured in the user equipment. The addition and deletion of SCell are performed by radio resource control (RRC) signaling. SCell is in a deactivate state immediately after it is configured in the user equipment. Therefore, SCell is activated into a communicable (schedulable) cell. As illustrated in FIG. 1, in CA of LTE Rel-10, a plurality of CCs are used under the control of the same base station.
In contrast, in Rel-12, dual connectivity has been proposed which performs simultaneous communication using CCs under the control of different base stations to achieve high throughput. That is, in the dual connectivity, user equipment performs communication, using the radio resources of two physically different base stations at the same time.
The dual connectivity is a kind of CA and is also called inter-base station carrier aggregation (Inter eNB CA). Master-eNB (MeNB) and Secondary-eNB (SeNB) are introduced. FIG. 2 illustrates an example of the dual connectivity. In the example illustrated in FIG. 2, MeNB communicates with the user equipment, using CC #1, and SeNB communicates with the user equipment, using CC #2. In this way, dual connectivity (hereinafter, referred to as DC) is achieved.
In DC, a bearer that is called a split bearer, namely, a logical packet path including branched paths, is defined. In the split bearer, MeNB performs the process of a packet data control protocol (PDCP) layer and both MeNB and SeNB perform the processes of a radio link control (RLC) layer, a media access control (MAC) layer, and a physical (PHY) layer. The use of the split bearer makes it possible to distribute the processing load on a base station and to enhance the throughput of data transmitted to user equipment UE.